////
This guide is maintained in the main Quarkus repository
and pull requests should be submitted there:
https://github.com/quarkusio/quarkus/tree/master/docs/src/main/asciidoc
////
= Quarkus - Contexts and Dependency Injection

include::./attributes.adoc[]
:numbered:
:sectnums:
:sectnumlevels: 4
:toc:

:numbered:
:sectnums:
:sectnumlevels: 4

Quarkus DI solution is based on the http://docs.jboss.org/cdi/spec/2.0/cdi-spec.html[Contexts and Dependency Injection for Java 2.0, window="_blank"] specification.
However, it is not a full CDI implementation verified by the TCK.
Only a subset of the CDI features is implemented - see also <<supported_features,the list of supported features>> and <<limitations,the list of limitations>>.

NOTE: Most of the existing CDI code should work just fine but there are some small differences which follow from the Quarkus architecture and goals.

[[bean_discovery]]
== Bean Discovery

Bean discovery in CDI is a complex process which involves legacy deployment structures and accessibility requirements of the underlying module architecture.
However, Quarkus is using a *simplified bean discovery*.
There is only single bean archive with the https://docs.jboss.org/cdi/spec/2.0/cdi-spec.html#default_bean_discovery[bean discovery mode `annotated`, window="_blank"] and no visibility boundaries.

The bean archive is synthesized from:

* the application classes,
* dependencies that contain a `beans.xml` descriptor (content is ignored),
* dependencies that contain a Jandex index - `META-INF/jandex.idx`,
* dependencies referenced by `quarkus.index-dependency` in `application.properties`,
* and Quarkus integration code.

Bean classes that don't have a http://docs.jboss.org/cdi/spec/2.0/cdi-spec.html#bean_defining_annotations[bean defining annotation, window="_blank"] are not discovered.
This behavior is defined by CDI.
But producer methods and fields and observer methods are discovered even if the declaring class is not annotated with a bean defining annotation (this behavior is different to what is defined in CDI).
In fact, the declaring bean classes are considered annotated with `@Dependent`. 

NOTE: Quarkus extensions may declare additional discovery rules. For example, `@Scheduled` business methods are registered even if the declaring class is not annotated with a bean defining annotation.

=== How to Generate a Jandex Index

A dependency with a Jandex index is automatically scanned for beans.
To generate the index just add the following to your `pom.xml`:

[source,xml]
----
<build>
  <plugins>
    <plugin>
      <groupId>org.jboss.jandex</groupId>
      <artifactId>jandex-maven-plugin</artifactId>
      <version>1.0.6</version>
      <executions>
        <execution>
          <id>make-index</id>
          <goals>
            <goal>jandex</goal>
          </goals>
        </execution>
      </executions>
    </plugin>
  </plugins>
</build>
----

If you can't modify the dependency, you can still index it by adding `quarkus.index-dependency` entries to your `application.properties`:

[source,properties]
----
quarkus.index-dependency.<name>.group-id=
quarkus.index-dependency.<name>.artifact-id=
quarkus.index-dependency.<name>.classifier=(this one is optional)
----

For example, the following entries ensure that the `org.acme:acme-api` dependency is indexed:

[source,properties]
----
quarkus.index-dependency.acme.group-id=org.acme <1>
quarkus.index-dependency.acme.artifact-id=acme-api <2>
----
<1> Value is a group id for a dependency identified by name `acme`.
<2> Value is an artifact id for a dependency identified by name `acme`.

== Private Members

Quarkus is designed with Substrate VM in mind.
One of the limitations is the usage of https://github.com/oracle/graal/blob/master/substratevm/LIMITATIONS.md#user-content-reflection[Reflection, window="_blank"].
Substrate VM does support reflective calls but for a price of a bigger native executable.
Quarkus must use reflection fallback to access private members.
That's why Quarkus users are encouraged __not to use private members__ in their beans.
This involves injection fields, constructors and initializers, observer methods, producer methods and fields, disposers and interceptor methods.

You can use for example package-private modifiers:

[source,java]
----
@ApplicationScoped
public class CounterBean {

    @Inject
    CounterService counterService; <1>
    
    void onMessage(@Observes Event msg) { <2>
    }
}
----
<1> A package-private injection field.
<2> A package-private observer method.

Or constructor injection:

[source,java]
----
@ApplicationScoped
public class CounterBean {

    private CounterService service;

    CounterBean(CounterService service) { <1>
      this.service = service;
    }
}
----
<1> A package-private constructor injection.


[[supported_features]]
== Supported Features

* Programming model
** Managed beans implemented by a Java class
*** `@PostConstruct` and `@PreDestroy` lifecycle callbacks
** Producer methods and fields, disposers
** Qualifiers
** Alternatives
** Stereotypes
* Dependency injection and lookup
** Field, constructor and initializer/setter injection
** Type-safe resolution 
** Programmatic lookup via `javax.enterprise.inject.Instance`
** Client proxies
** Injection point metadata
* Scopes and contexts
** `@Dependent`, `@ApplicationScoped`, `@Singleton`, `@RequestScoped` and `@SessionScoped`
** Custom scopes and contexts
* Interceptors
** Business method interceptors: `@AroundInvoke`
** Interceptors for lifecycle event callbacks: `@PostConstruct`, `@PreDestroy`, `@AroundConstruct`
* Events and observers, including asynchronous events

[[limitations]]
== Limitations

* `@ConversationScoped` is not supported
* Decorators are not supported
* Portable Extensions are not supported
* `BeanManager` - only the following methods are implemented: `getBeans()`, `createCreationalContext()`, `getReference()`, `getInjectableReference()` , `resolve()`, `getContext()`, `fireEvent()`, `getEvent()` and `createInstance()`
* Specialization is not supported
* `beans.xml` descriptor content is ignored
* Passivation and passivating scopes are not supported
* Interceptor methods on superclasses are not implemented yet
* `BEFORE_COMPLETION`, `AFTER_COMPLETION`, `AFTER_FAILURE` and `AFTER_SUCCESS` transactional observers are not implemented yet

== Build Time Extension Points

[[portable_extensions]]
=== Portable Extensions

Quarkus incorporates build-time optimizations in order to provide instant startup and low memory footprint.
The downside of this approach is that CDI Portable Extensions cannot be supported.
Nevertheless, most of the functionality can be achieved using Quarkus link:writing-extensions.html[extensions].

=== Additional Bean Defining Annotations

As described in <<bean_discovery>> bean classes that don’t have a bean defining annotation are not discovered.
However, `BeanDefiningAnnotationBuildItem` can be used to extend the set of default bean defining annotations (`@Dependent`, `@Singleton`, `@ApplicationScoped`, `@RequestScoped` and `@Stereotype` annotations):

[source,java]
----
@BuildStep
BeanDefiningAnnotationBuildItem additionalBeanDefiningAnnotation() {
    return new BeanDefiningAnnotationBuildItem(DotName.createSimple("javax.ws.rs.Path")));
}
----

NOTE: Bean registrations that are result of a `BeanDefiningAnnotationBuildItem` are unremovable by default. See also <<remove_unused_beans>>.

=== Resource Annotations

`ResourceAnnotationBuildItem` is used to specify resource annotations  that make it possible to resolve non-CDI injection points, such as Java EE resources.

NOTE: An integrator must also provide a corresponding `io.quarkus.arc.ResourceReferenceProvider` implementation. 

[source,java]
----
@BuildStep
void setupResourceInjection(BuildProducer<ResourceAnnotationBuildItem> resourceAnnotations, BuildProducer<GeneratedResourceBuildItem> resources) {
    resources.produce(new GeneratedResourceBuildItem("META-INF/services/io.quarkus.arc.ResourceReferenceProvider",
        JPAResourceReferenceProvider.class.getName().getBytes()));
    resourceAnnotations.produce(new ResourceAnnotationBuildItem(DotName.createSimple(PersistenceContext.class.getName())));
}
----

=== Additional Beans

`AdditionalBeanBuildItem` is used to specify additional bean classes to be analyzed during discovery.
Additional bean classes are transparently added to the application index processed by the container.

[source,java]
----
@BuildStep
List<AdditionalBeanBuildItem> additionalBeans() {
     return Arrays.asList(
          new AdditionalBeanBuildItem(SmallRyeHealthReporter.class),
          new AdditionalBeanBuildItem(HealthServlet.class));
}
----

NOTE: A bean registration that is a result of an `AdditionalBeanBuildItem` is removable by default. See also <<remove_unused_beans>>.

=== Synthetic Beans

Sometimes it is very useful to register a synthetic bean, i.e. a bean that doesn't need to have a corresponding java class.
In CDI this could be achieved using `AfterBeanDiscovery.addBean()` methods.
In Quarkus we produce a `BeanRegistrarBuildItem` and leverage the `io.quarkus.arc.processor.BeanConfigurator` API to build a synthetic bean definition.

[source,java]
----
@BuildStep
BeanRegistrarBuildItem syntheticBean() {
     return new BeanRegistrarBuildItem(new BeanRegistrar() {

            @Override
            public void register(RegistrationContext registrationContext) {
                 registrationContext.configure(String.class).types(String.class).qualifiers(new MyQualifierLiteral()).creator(mc -> mc.returnValue(mc.load("foo"))).done();
            }
        }));
}
----

NOTE: The output of a `BeanConfigurator` is recorded as bytecode. Therefore there are some limitations in how a synthetic bean instance is created. See also `BeanConfigurator.creator()` methods.

If an extension needs to produce other build items during the "bean registration" phase it should use the `BeanRegistrationPhaseBuildItem` instead.
The reason is that injected objects are only valid during a `@BuildStep` method invocation.

[source,java]
----
@BuildStep
void syntheticBean(BeanRegistrationPhaseBuildItem beanRegistrationPhase,
            BuildProducer<MyBuildItem> myBuildItem, 
            BuildProducer<BeanConfiguratorBuildItem> beanConfigurators) {
   beanConfigurators.produce(new BeanConfiguratorBuildItem(beanRegistrationPhase.getContext().configure(String.class).types(String.class).qualifiers(new MyQualifierLiteral()).creator(mc -> mc.returnValue(mc.load("foo")))));
   myBuildItem.produce(new MyBuildItem());
}
----

NOTE: See `BeanRegistrationPhaseBuildItem` javadoc for more information.


=== Annotation Transformations

A very common task is to override the annotations found on the bean classes.
For example you might want to add an interceptor binding to a specific bean class.
Here is how to do it - use the `AnnotationsTransformerBuildItem`:

[source,java]
----
@BuildStep
AnnotationsTransformerBuildItem transform() {
    return new AnnotationsTransformerBuildItem(new AnnotationsTransformer() {

        public boolean appliesTo(org.jboss.jandex.AnnotationTarget.Kind kind) {
            return kind == org.jboss.jandex.AnnotationTarget.Kind.CLASS;
        }

        public void transform(TransformationContext context) {
            if (contex.getTarget().asClass().name().toString().equals("com.foo.Bar")) {
                context.transform().add(MyInterceptorBinding.class).done();
            }
        }
    });
}
----

=== Additional Interceptor Bindings

In rare cases it might be handy to programmatically register an existing annotation as interceptor binding.
This is similar to what pure CDI achieves through `BeforeBeanDiscovery#addInterceptorBinding()`.
Though here we are going to use `InterceptorBindingRegistrarBuildItem` to get it done.
Note that you can register multiple annotations in one go:

[source,java]
----
@BuildStep
InterceptorBindingRegistrarBuildItem addInterceptorBindings() {
    InterceptorBindingRegistrarBuildItem additionalBindingsRegistrar = new InterceptorBindingRegistrarBuildItem(new InterceptorBindingRegistrar() {
        @Override
        public Collection<DotName> registerAdditionalBindings() {
            Collection<DotName> result = new HashSet<>();
            result.add(DotName.createSimple(MyAnnotation.class.getName()));
            result.add(DotName.createSimple(MyOtherAnnotation.class.getName()));
            return result;
        }
    });
    return additionalBindingsRegistrar;
}
----

=== Injection Point Transformation

Every now and then it is handy to be able to change qualifiers of an injection point programmatically.
You can do just that with `InjectionPointTransformerBuildItem`.
The following sample shows how to apply transformation to injection points with type `Foo` that contain qualifier `MyQualifier`:

[source,java]
----
@BuildStep
InjectionPointTransformerBuildItem transform() {
    return new InjectionPointTransformerBuildItem(new InjectionPointsTransformer() {

        public boolean appliesTo(Type requiredType) {
            return requiredType.equals(Type.create(DotName.createSimple(Foo.class.getName()), Type.Kind.CLASS));
        }

        public void transform(TransformationContext transformationContext) {
            if (transformationContext.getQualifiers().stream()
                    .anyMatch(a -> a.name().equals(DotName.createSimple(MyQualifier.class.getName())))) {
                transformationContext.transform().removeAll()
                        .add(DotName.createSimple(MyOtherQualifier.class.getName()))
                        .done();
            }
        }
    });
}
----

=== Bean Deployment Validation

Once the bean deployment is ready an extension can perform additional validations and inspect the found beans, observers and injection points.
Register a `BeanDeploymentValidatorBuildItem`:

[source,java]
----
@BuildStep
BeanDeploymentValidatorBuildItem beanDeploymentValidator() {
    return new BeanDeploymentValidatorBuildItem(new BeanDeploymentValidator() {
         public void validate(ValidationContext validationContext) {
             for (InjectionPointInfo injectionPoint : validationContext.get(Key.INJECTION_POINTS)) {
                 System.out.println("Injection point: " + injectionPoint);
             }
         }
    });
}
----

NOTE: See also `io.quarkus.arc.processor.BuildExtension.Key` to discover the available metadata.

If an extension needs to produce other build items during the "validation" phase it should use the `ValidationPhaseBuildItem` instead.
The reason is that injected objects are only valid during a `@BuildStep` method invocation.

[source,java]
----
@BuildStep
void validate(ValidationPhaseBuildItem validationPhase,
            BuildProducer<MyBuildItem> myBuildItem, 
            BuildProducer<ValidationErrorBuildItem> errors) {
   if (someCondition) {
     errors.produce(new ValidationErrorBuildItem(new IllegalStateException()));
     myBuildItem.produce(new MyBuildItem());
   }
}
----

NOTE: See `ValidationPhaseBuildItem` javadoc for more information.


=== Custom Contexts

An extension can register a custom `InjectableContext` implementation by means of a `ContextRegistrarBuildItem`:

[source,java]
----
@BuildStep
ContextRegistrarBuildItem customContext() {
    return new ContextRegistrarBuildItem(new ContextRegistrar() {
         public void register(RegistrationContext registrationContext) {
            registrationContext.configure(CustomScoped.class).normal().contextClass(MyCustomContext.class).done();
         }
    });
}
----

If an extension needs to produce other build items during the "context registration" phase it should use the `ContextRegistrationPhaseBuildItem` instead.
The reason is that injected objects are only valid during a `@BuildStep` method invocation.

[source,java]
----
@BuildStep
void addContext(ContextRegistrationPhaseBuildItem contextRegistrationPhase,
            BuildProducer<MyBuildItem> myBuildItem, 
            BuildProducer<ContextConfiguratorBuildItem> contexts) {
   contexts.produce(new ContextConfiguratorBuildItem(contextRegistrationPhase.getContext().configure(CustomScoped.class).normal().contextClass(MyCustomContext.class)));
   myBuildItem.produce(new MyBuildItem());
}
----

NOTE: See `ContextRegistrationPhaseBuildItem` javadoc for more information.


[[build_metadata]]
=== Available Build Time Metadata

Any of the above extensions that operates with `BuildExtension.BuildContext` can leverage certain build time metadata that are generated during build.
The built-in keys located in `io.quarkus.arc.processor.BuildExtension.Key` are:

* `ANNOTATION_STORE`
** Contains an `AnnotationStore` that keeps information about all `AnnotationTarget` annotations after application of annotation transformers
* `INJECTION_POINTS`
** `List<InjectionPointInfo>` containing all injection points
* `BEANS`
** `List<BeanInfo>` containing all beans
* `OBSERVERS`
** `List<ObserverInfo>` containing all observers
* `SCOPES`
** `List<ScopeInfo>` containing all scopes, including custom ones
* `QUALIFIERS`
** `Map<DotName, ClassInfo>` containing all qualifiers
* `INTERCEPTOR_BINDINGS`
** `Map<DotName, ClassInfo>` containing all interceptor bindings
* `STEREOTYPES`
** `Map<DotName, ClassInfo>` containing all stereotypes

To get hold of these, simply query the extension context object for given key.
Note that these metadata are made available as build proceeds which means that extensions can only leverage metadata that were build before they are invoked.
If your extension attempts to retrieve metadata that wasn't yet produced, `null` will be returned.
Here is a summary of which extensions can access which metadata:

* `AnnotationsTransformer`
** Shouldn't rely on any metadata as this is one of the first CDI extensions invoked
* `ContextRegistrar`
** Has access to `ANNOTATION_STORE`
* `InjectionPointsTransformer`
** Has access to `ANNOTATION_STORE`, `QUALIFIERS`, `INTERCEPTOR_BINDINGS`, `STEREOTYPES`
* `BeanRegistrar`
** Has access to all build metadata
* `BeanDeploymentValidator`
** Has access to all build metadata


[[remove_unused_beans]]
== Removing Unused Beans

The container attempts to remove all unused beans during build by default.
This optimization can be disabled by setting `quarkus.arc.remove-unused-beans` to `none` or `false`.

An unused bean:

* is not a built-in bean or an interceptor,
* is not eligible for injection to any injection point,
* is not excluded by any extension,
* does not have a name,
* does not declare an observer,
* does not declare any producer which is eligible for injection to any injection point,
* is not directly eligible for injection into any `javax.enterprise.inject.Instance` or `javax.inject.Provider` injection point

This optimization applies to all forms of bean declarations: bean class, producer method, producer field.

Users can instruct the container to not remove any of their specific beans (even if they satisfy all the rules specified above) by annotating them with `io.quarkus.arc.Unremovable`.
This annotation can be placed on the types, producer methods, and producer fields.
 
Furthermore, extensions can eliminate possible false positives by producing `UnremovableBeanBuildItem`.

Finally, Quarkus provides a middle ground for the bean removal optimization where application beans are never removed whether or not they are unused,
while the optimization proceeds normally for non application classes. To use this mode, set `quarkus.arc.remove-unused-beans` to `fwk` or `framework`.

When using the dev mode (running `mvn clean compile quarkus:dev`), you can see more information about which beans are being removed
by enabling additional logging via the following line in your `application.properties`.

----
quarkus.log.category."io.quarkus.arc.processor".level=DEBUG
----

[[default_beans]]
== Default Beans

Quarkus adds a capability that CDI currently does not support which is to conditionally declare a bean if no such bean has been declared in the typical ways Quarkus supports.
This is done using the `@io.quarkus.arc.DefaultBean` annotation and is best explained with an example.

Say there is a Quarkus extension that among other things declares a few CDI beans like the following code does:

[source,java]
----
@Dependent
public class TracerConfiguration {

    @Produces
    public Tracer tracer(Reporter reporter, Configuration configuration) {
        return new Tracer(reporter, configuration);
    }

    @Produces
    @DefaultBean
    public Configuration configuration() {
        // create a Configuration
    }

    @Produces
    @DefaultBean
    public Reporter reporter(){
        // create a Reporter
    }
}
----

The idea is that the extension auto-configures things for user, eliminating a lot of boilerplate - we can just `@Inject` a `Tracer` wherever it is needed.
Now imagine that in our application we would like to utilize the configured `Tracer`, but we need to customize it a little, for example by providing a custom `Reporter`.
The only thing that would be needed in our application would be something like the following:


[source,java]
----
@Dependent
public class CustomTracerConfiguration {

    @Produces
    public Reporter reporter(){
        // create a custom Reporter
    }
}
----

`@DefaultBean` allows extensions (or any other code for that matter) to provide defaults while backing off if beans of that type are supplied in any
way Quarkus supports.
